The present invention relates to the manufacture of integrated circuits, and more specifically, to a method of forming fluorinated silicate glass (xe2x80x9cFSGxe2x80x9d) films with good adhesion to silicon nitride in a high-density-plasma chemical-vapor-deposition (xe2x80x9cHDP-CVDxe2x80x9d) system.
In conventional integrated circuit fabrication, circuit elements are formed by etching a pattern of gaps in a layer of metal such as aluminum. The gaps are then filled with a dielectric such as silicon dioxide. Copper is poised to take over as the main on-chip conductor for all types of integrated circuits because of its lower resistance when compared to conventional aluminum alloys. Because it is difficult to etch copper, however, damascene processes have been developed for fabricating copper based integrated circuits. In damascene processes, dielectric layers are deposited and then etched to form gaps that are subsequently filled with copper.
Fluorine-doped silicon oxide, also known as fluorosilicate glass, is an attractive solution to replace conventional silicon dioxide as an intermetal dielectric for damascene structures. An FSG film can be deposited in conventional HDP-CVD systems, which have been widely used for undoped silicate glass (USG) and FSG dielectrics in aluminum interconnects. FSG has a good process scheme in terms of reliability, stability, and throughput. Furthermore, the electrical performance of integrated circuits can be improved because of the lower dielectric constant of FSG (3.4 compared to 4.1 for conventional silicon oxides). The lower dielectric constant reduces the capacitance between metal lines in the same layer and reduces cross talk across layers.
Dielectric films used in damascene processes utilize a layer known as an etch stop to provide for selective etching of the film. Silicon nitride (SixNy) is commonly used as an etch stop in damascene applications, for example when forming vias between layers containing metal lines. In the past, there have been problems in obtaining good adhesion between the silicon nitride and an underlying or overlying layer of FSG. Specifically, the FSG tends to outgas at temperatures of about 450xc2x0 C. forming xe2x80x9cbubblesxe2x80x9d in an overlying SixNylayer. The bubbles lead to delamination of the SixNy. Previous attempts to improve the adhesion by, for example, reducing the fluorine content in the FSG merely postpone the delamination. When FSG films are deposited on a silicon nitride barrier layer in damascene or dual damascene applications, failure to integrate the FSG with the barrier layer poses a significant obstacle in the widespread acceptance of FSG as an adequate low-k dielectric material.
Therefore, a need exists in the art for a method of depositing an FSG film with improved adhesion to an overlying or underlying layer of silicon nitride.
The disadvantages associated with the prior art are overcome by a method of formation of the damascene FSG film with good adhesion to silicon nitride. In one embodiment of the present invention, a multilayer FSG film is deposited as part of a film stack that includes an adjacent silicon nitride layer. The multilayer FSG film includes an interfacial FSG layer and a bulk FSG layer. The xe2x80x9cinterfacial FSG layerxe2x80x9d refers to the portion of the FSG film deposited adjacent to the silicon nitride layer. Thus, the interfacial part of the FSG film refers to the topmost portion if silicon nitride is to be deposited on top of the FSG and it refers to the bottom portion if the FSG is to be deposited on top of silicon nitride. It is not required that the xe2x80x9cbulk FSG layerxe2x80x9d be thicker than the interfacial layer, although it may be in some embodiments. The multilayer FSG film is deposited by flowing a gaseous mixture comprising flows of silane, a gas that contains both fluorine and silicon, and a gas that contains oxygen to a process chamber. The gas that contains both fluorine and silicon is preferably SiF4, and the gas that contains oxygen is preferably O2. It is also preferable that the gaseous mixture comprise a flow of inert gas, such as Ar, to promote gas dissociation. A plasma, preferably a high-density plasma is generated from the gaseous mixture, and the bulk portion o f an FSG layer is deposited on the substrate using the plasma. The silane flow is then terminated during deposition of the interfacial part of the FSG film. By removing the SiH4 from the deposition of the interfacial part of the FSG film, less hydrogen is incorporated into the film in the interfacial region and adhesion to overlying or underlying silicon nitride is improved. In other embodiments, a multilayer FSG film that includes bottom and top interfacial layers sandwiched around a bulk FSG layer is formed.
In another embodiment, a pure SiF4-only fluorinated oxide (xe2x80x9cSOFOxe2x80x9d) layer is deposited without having any portion of the film formed with silane. In this embodiment, a gaseous mixture is provided that comprises flows of a gas that contains both fluorine and silicon and a gas that contains oxygen, but not containing a silane, for the entire deposition. It is again preferred that gaseous mixture comprise a flow of inert gas, such as Ar, and that the gas containing both fluorine and silicon be SiF4 and that the gas containing oxygen be O2. In further embodiments, the SOFO layer is deposited as part of a copper damascene process on a barrier layer, preferably silicon nitride, previously formed on the substrate. In still other embodiments, the SOFO layer is etched, ashed, and metallized. The ashing is preferably performed with an oxygen, ammonia, or mixed oxygen-ammonia chemistry, and the metallization layer is preferably formed of Ta or TaN.
The methods of the present invention may be embodied in a computer-readable storage medium having a computer-readable program embodied therein for directing operation of a substrate processing system. Such a system may include a process chamber, a plasma generation system, a substrate holder, a gas delivery system, and a system controller. The computer-readable program includes instructions for operating the substrate processing system to form a thin film on a substrate disposed in the processing chamber in accordance with the embodiments described above.